Let's say I have around 500~600 attack.
I'm using only Legit Items, no customs at all.

I'm using mainly Double Strafing, I don't hitlock or use Mines, just Ankle Snare, if the situation requires using them.

What's better?

Bow Thimble or Glove
Sniping Suit or something else?
What about Garment? which one should I use?
Composite bow with 4xAK or 4xSkeleton Worker / 4xMinourus / ???x Something else?
Maybe some other kind of Bow? Since Comp Bow is level 2 it doesn't add as much damage as a Level 3~4 Bow.

1. depends probably the glove for the easier slot
2. something with +3 to dex
3. something that adds to your atk or dex or else it doesnt matter since you dont care about agi
4. +10 compo bow with a mixture of 1 of each card is "usually the best" result in terms of damage
5. +10 compo bow with 4 modifier cards should give you most damage.
6. yes archer combo

as for the bosses you wont do well since your spam rate for DS is based on your attack speed and since you dont intend to "hitlock" you will probably only get about 2-3 ds in before you are dead.

as for setups go google ratemyserver and do some research you will learn a lot about the game.

Then scratch the orc archer bow and stick with a +10 Composite Bow with anti racial cards.
1 Bow for Bio Labs could be Double Bloody Double Boned (but I once read something in ROEmpire Forums about Triple Bloody Boned, but I can't remember much).
I don't know much about MvP Bow build (since I never got the chance to MvP on a sniper during my legit days. XD).

About the lower head gear, Romantic Flower?

Middle gear, if slotted sunglasses are available in Eir, then maybe that with a Vanberk.

SORRY if my life doesn't revolve around PvP and other in-depth technical stuff about the game.Sucks to have a RL.

I really like it when a random person steps in a thread and posts something without even reading and comprehending the topic and the discussion.

You should get the Archer Card Combo, which if you didn't know, includes Cruiser, Anolian, Merman, Alligator, and Dragon Tail. Get 3 AK cards for a +10 Comp bow since Cruiser card will take up one of those slots. If you are looking to get a lot of crits out of your damage, I would suggest using a Sniping Suit. If you just want more damage all together, get a +3 Dex Tights[1]. Garment could always be a slotted Crest of the Rider or Valk Mant. Whichever you prefer. Crest of the Rider would be better if you were using slotted Black Leather Boots. Binoculars, Apple of Archer for the headgears since no donates. I think that's about all the advice I can offer.